1. Field of the Invention
The present invention relates to a lifting device and, more particularly, to a lifting device for a chair.
2. Description of the Related Art
A first conventional lifting device 1 for a chair in accordance with the prior art shown in FIGS. 5 and 6 comprises an upright support post 14, a C-shaped pressing tube 11 mounted on the support post 14, a stepping ring 15 having a central mounting sleeve 10 which is mounted on the pressing tube 11 and has a peripheral wall provided with a screw bore 12 connected to the pressing tube 11, and a threaded locking knob 13 screwed into the screw bore 12 of the mounting sleeve 10 and pressing the pressing tube 11 to lock the pressing tube 11 and the mounting sleeve 10 of the stepping ring 15 onto the support post 14. However, the locking knob 13 forces the pressing tube 11 in a single point so that the pressing tube 11 is easily worn out or broken due to a stress concentration.
A second conventional lifting device 2 for a chair in accordance with the prior art shown in FIGS. 7 and 8 comprises an upright support post 27, a tapered pressing ring 23 mounted on the support post 27 and having an upper end provided with an outer thread 28 and a lower end provided with a through hole 24 to receive a rubber plate 25 and a tapered pressing block 26, and a stepping ring having a central mounting sleeve 20 which is mounted on the pressing ring 23 and has an upper end provided with an inner thread 22 screwed onto the outer thread 28 of the pressing ring 23 and a lower end provided with a ramp 21 pressing the pressing block 26. Thus, the inner thread 22 of the mounting sleeve 20 is screwed onto the outer thread 28 of the pressing ring 23 so that when the mounting sleeve 20 is rotated relative to the pressing ring 23, the ramp 21 of the mounting sleeve 20 is moved to press the pressing block 26 and the rubber plate 25 toward the support post 27 to lock the pressing ring 23 onto the support post 27. However, dust or dirt easily enters the mounting sleeve 20 of the stepping ring to contaminate the pressing ring 23 and the pressing block 26, thereby affecting operation of the pressing ring 23 and the pressing block 26.